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From:   Josh Don <joshdon@google.com>
Date:   Wed, 11 Oct 2023 16:55:35 -0700
Message-ID: <CABk29NvHx2saNLdYmQgt31R8W28p7=GUtXiiupgE5czXRBAx5g@mail.gmail.com>
Subject: Re: [PATCH RFC] cpumask: Randomly distribute the tasks within
 affinity mask
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ankit Jain <ankitja@vmware.com>, yury.norov@gmail.com,
        andriy.shevchenko@linux.intel.com, linux@rasmusvillemoes.dk,
        qyousef@layalina.io, pjt@google.com, bristot@redhat.com,
        vschneid@redhat.com, linux-kernel@vger.kernel.org,
        namit@vmware.com, amakhalov@vmware.com, srinidhir@vmware.com,
        vsirnapalli@vmware.com, vbrahmajosyula@vmware.com,
        akaher@vmware.com, srivatsa@csail.mit.edu
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Precedence: bulk

Hey Peter,

> +static struct cpumask *root_domain_allowed(struct cpumask *newmask,
> +                                          struct cpumask *scratch,
> +                                          struct cpumask *valid)
> +{
> +       struct root_domain *rd;
> +       struct cpumask *mask;
> +       struct rq *rq;
> +
> +       int first = cpumask_first_and(newmask, valid);
> +       if (first >= nr_cpu_ids)
> +               return NULL;

This picks the first arbitrarily, but isn't it possible for the mask
to span both isolated and non-isolated cpus? In which case, the rest
of this just boils down to chance (ie. whatever the span happens to be
for the first cpu here)?